The processing of oil shale to obtain usable oil products has increased in recent years as oil shale has become a more feasible source of energy. In conjunction with the processing of the oil shale, one of the end products generated is spent shale. Spent shale can be defined as including the characteristics of a low BTU, low volatile content, low carbon, high ash fuel. Spent shale typically comprises a wide range of particle sizes, e.g. fines to three inches. Because useful energy remains in spent shale, it would be desirable to be able to utilize that energy. However, known combusting systems are not able efficiently to handle and to combust the spent shale because of the relatively wide range of spent shale particle sizes.
In known entrained or fluidized bed combustors, relatively small particles (less than one inch) are effectively handled but such combustors are inadequate for relatively larger particles. In another known type of combustor, relatively large particles (1-4 inches) are combusted but such a system cannot tolerate any significant amount of smaller sized particles or fines due to the unacceptably high pressure drop created by such particles.
In order to treat the spent shale to obtain useful energy therefrom, it has been necessary first to separate the spent shale particles into large and small size particles before introducing the separated larger particles into a combustor adapted to handle such particles and separately introducing the smaller particles into another combustor adapted to handle these smaller particles and fines. Alternatively, it has been suggested to subject the larger particles to a grinding operation so that smaller size particles are formed. However, any separation process involves additional processing time and results in further expense without any energy being obtained during this process step. The grinding operation is also disadvantageous. Because the spent shale is generated at a relatively high temperature (800.degree. F.), the grinding step causes a substantial removal of heat from the spent shale by contact with the grinding equipment. Since it is highly desirable to maintain the spent shale at this high temperature, the grinding operation results in a wasteful loss of heat.
Because of the foregoing problems associated with obtaining energy from spent shale, it is not uncommon simply to dispose of the spent shale, instead of trying to obtain any further useful energy therefrom. Even this disposal process has drawbacks due to the high temperature of the generated spent shale and the handling thereof, as well as the considerable quantities of the spent shale generated during the processing of oil shale. As a consequence, an apparatus and method which economically and efficiently combusts different size particles of spent shale, without the loss of substantial heat present in the generated spent shale, would provide significant benefits over known systems.